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  1. 1. ENTC 4350 Pacemakers
  2. 2. <ul><li>A pacemaker is a prosthetic device for the heart, first conceived in 1932 by Albert S. Hymen, an American cardiologist. </li></ul><ul><ul><li>In 1952 the pacemaker was used clinically by Paul M. Zoll as an external device. </li></ul></ul>
  3. 3. <ul><li>With the advent of solid-state circuitry in the early 1960s, it was made into a battery-operated prosthesis that was implantable into the patient. </li></ul><ul><ul><li>Credit for the implantable pacemaker is given to the American physicians William Chardack and Andrew Gage and to the engineer Wilson Greatbatch. </li></ul></ul>
  4. 4. <ul><li>Other heart prostheses, or spare parts, include coronary bypass vessels and artificial heart valves. </li></ul><ul><ul><li>An especially innovative recent heart prosthesis is the artificial heart, the best known example of which is the Jarvik-7, designed by Robert K. Jarvik and implanted into Barney Clark by William DeVries in 1982. </li></ul></ul><ul><ul><ul><li>Another implantable artificial heart was developed in 1985 at Penn State University by a team headed by William Pierce. </li></ul></ul></ul>
  5. 5. <ul><li>A pacemaker is a prosthesis specifically for the sinoatrial (SA) node of the heart. </li></ul><ul><ul><li>The SA node may become ineffective for several reasons, among them: </li></ul></ul><ul><ul><ul><li>the SA node tissue or atrium may become diseased; or </li></ul></ul></ul><ul><ul><ul><li>the path of the heart depolarization—specifically, the atrioventricular (AV) node from the atrium to the ventricles—may become diseased, producing a heart block. </li></ul></ul></ul>
  6. 6. <ul><li>Furthermore, bradycardia, a slowing of the heart rate generally to below 50 or 60 beats per minute (bpm), may develop because of aging or other reasons. </li></ul><ul><ul><li>These diseases may be treated either with a pacemaker or with medicine, depending upon the case. </li></ul></ul>
  7. 7. <ul><li>In the case that the SA node fails to pace the heart properly, the ventricles may beat at their own self-paced rate, normally about 40 bpm. </li></ul><ul><ul><li>At this heart rate, a patient may survive, but may not be able to function normally. </li></ul></ul>
  8. 8. <ul><li>Because the pacemaker is battery-operated and surgically implanted, battery lifetime is one of the most important considerations. </li></ul><ul><ul><li>The lifetime is determined primarily by the stimulus requirements, as well as the current caused by the pacemaker circuitry. </li></ul></ul>
  9. 9. <ul><li>The use of complementary metal-oxide semiconductor (CMOS) integrated circuits has dramatically reduced the current drain, but the stimulus requirements are determined by physiology and cannot be reduced effectively. </li></ul>
  10. 10. <ul><li>As is usually the case with physiological stimuli, there is a curve of stimulus intensity versus duration associated with the physiological response of heart depolarization. </li></ul>
  11. 11. <ul><li>The figure shows the stimulus voltage, V s , at the tissue-electrode interface. </li></ul>V s (V) Time, T D (ms)
  12. 12. <ul><li>It has a stimulus duration T D , measured in milliseconds. </li></ul><ul><ul><li>Such curves depend upon the electrode-heart resistance, R H , which may range from 100 to 1400  . </li></ul></ul>
  13. 13. <ul><li>The value of R H may change over time because of tissue scarring at the electrode-tissue interface. </li></ul><ul><ul><li>In order to produce a stimulus pulse, it is necessary to deliver energy to the electrode with a pacemaker circuit. </li></ul></ul>
  14. 14. <ul><li>A pacemaker in its simplest configuration is essentially a battery-operated digital pulse generator. </li></ul>
  15. 15. <ul><li>A digital pulse has a voltage V s that may be made variable to allow adjustments in the energy, E P , delivered by the pacemaker to the heart during each pulse. </li></ul>
  16. 16. <ul><li>During the pulse duration, T D , the stimulus voltage drives energy into the heart. </li></ul><ul><ul><li>When the pulse is OFF, it causes an energy drain given by V s I D T , where T is the time period between successive pulses, and I D is the current drain on the battery when the pulse is OFF. </li></ul></ul>
  17. 17. <ul><li>Therefore, the energy delivered by the pacemaker during each pulse is given as </li></ul>
  18. 18. EXAMPLE <ul><li>Using the figure, compute the energy per pulse when the pacemaker pulse width is 0.5 ms, the circuit-current drain is 1  A, the heart-electrode resistance is 200  , and the heart rate is 70 bpm. </li></ul>
  19. 19. SOLUTION <ul><li>From the figure, V s = 1.8 V. Also, T = (60/70) s. Then, </li></ul><ul><ul><li>Thus the energy used for each pulse is </li></ul></ul><ul><ul><li>EP = 9.643  J/pulse </li></ul></ul>
  20. 20. Pacemaker Batteries <ul><li>Battery-operated equipment is convenient in many applications other than pacemakers because it can be used without a power cord, and it is safer because leakage currents are not usually present. </li></ul><ul><ul><li>The disadvantage is that batteries are relatively large and of limited energy-storage capability. </li></ul></ul><ul><ul><ul><li>Even so, the energy demand of the pacemaker is such that batteries with lifetimes between five and ten years are available. </li></ul></ul></ul>
  21. 21. <ul><li>Mercury cells with two-year lifetimes, used in pacemakers in the past, have been made obsolete by lithium iodide cells which can last as long as 15 years before they need to be replaced. </li></ul><ul><ul><li>Nuclear pacemaker batteries have been used to extend battery lifetimes to over 20 years, even for dual-chamber pacemakers that use higher amounts of battery power. </li></ul></ul>
  22. 22. <ul><li>Nuclear batteries pose an environmental hazard, however, because in an accident the radioactive material could be released into the environment. </li></ul><ul><ul><li>Nuclear batteries are being considered for artificial implanted hearts also, because of the potential for high energy storage, but this research is only beginning. </li></ul></ul>
  23. 23. <ul><li>Rechargeable batteries are not widely used for low power pacemaker application, since their shelf life is no longer than that of a lithium iodide pacemaker battery in normal use. </li></ul>
  24. 24. <ul><li>The lifetime of a storage battery depends on both its ampere-hour (A-H) rating and its shelf life. </li></ul><ul><ul><li>Shelf life is limited self-discharge of the battery due to internal leakage currents, particle migration, formation of insulating layers, and internal shorts. </li></ul></ul>
  25. 25. <ul><li>An illustrative example of a battery A-H rating versus its current drain is given in the figure. </li></ul>
  26. 26. <ul><li>At high current drain, polarization of the metal electrolyte boundary increases the internal resistance of the battery and decreases the A-H rating. </li></ul>
  27. 27. <ul><li>Implantable batteries are usually encased in metal. </li></ul><ul><ul><li>If they become too hot, such as when shorted, the case may rupture. </li></ul></ul><ul><ul><ul><li>Pacemaker design should ensure that the case is strong enough to contain such a rupture and prevent toxic materials from entering the body of the patient. </li></ul></ul></ul>
  28. 28. Illustrative Pacemaker Characteristics <ul><li>The pacemaker consists of three major components: </li></ul><ul><ul><li>the lead wire, </li></ul></ul><ul><ul><li>the electronic pulsing circuit, and </li></ul></ul><ul><ul><li>the battery. </li></ul></ul>
  29. 29. <ul><li>The lead can cause a failure due to metal fatigue, introduced by the motion and beating of the heart. </li></ul><ul><ul><li>To avoid such fatigue, the lead may be constructed by winding platinum ribbon around polyester yarn. </li></ul></ul><ul><ul><ul><li>Each lead may have three such wires for redundancy. </li></ul></ul></ul>
  30. 30. <ul><li>The pacemaker electrode must make a secure contact with t he heart for several years. </li></ul>
  31. 31. <ul><li>To ensure this, two methods of implantation are used under the following classifications: </li></ul><ul><ul><li>(1) endocardial lead, in which the pacemaker lead is inserted through a major vein through a catheter guide into the right ventricle of the heart; and </li></ul></ul><ul><ul><li>(2) epicardial lead, in which the pacemaker electrode is sutured to the external wall of the heart during open-heart surgery , and a wire electrode is thereby secured into the tissue. </li></ul></ul>
  32. 32. <ul><li>For endocardial lead implantation the electrode may be attached with tines. </li></ul>
  33. 33. <ul><li>The tines are pushed into the Purkinje muscle fibers of the ventricle and latch themselves in place. </li></ul><ul><ul><li>The porous electrode tip minimizes motion between the tip and the tissue so as to reduce the scar tissue buildup. </li></ul></ul><ul><ul><ul><li>This tends to keep the contact resistance low. </li></ul></ul></ul>
  34. 34. <ul><li>The electrode may also be held in place with a helical wire that is screwed into the tissues with a twisting motion. </li></ul>
  35. 35. <ul><li>In this case a bipolar electrode a few centimeters behind the contact electrode serves as a return path for current to the pacemaker. </li></ul>
  36. 36. <ul><li>In the unipolar pacemaker lead, the second electrode is eliminated, and the return conductive path to the pacemaker is made through body fluids. </li></ul><ul><ul><li>A unipolar lead electrode may also be held in place by either sutures, tines, or a helical wire. </li></ul></ul>
  37. 37. <ul><li>The electrode-muscle contact can change after a time because of </li></ul><ul><ul><li>(1) polarization by ionic current flow; </li></ul></ul><ul><ul><li>(2) tissue and scar growth; or </li></ul></ul><ul><ul><li>(3) mechanical motion of the heart. </li></ul></ul>
  38. 38. <ul><li>A symptom of such change may be an increased electrode impedance. </li></ul><ul><ul><li>The problem may be fixed by increasing the pulse voltage from the pacemaker or by lengthening its duration. </li></ul></ul><ul><ul><ul><li>Loss of contact altogether may require surgical reimplantation. </li></ul></ul></ul>
  39. 39. PROGRAMMABLE PACEMAKERS <ul><li>The implantable pacemaker is presented as a battery-powered, digital pulse generator, and it may be considered an asynchronous type of unit. </li></ul><ul><ul><li>Other types of pacemaker include the R-wave synchronous, R-wave inhibited, and P-wave synchronous pacemakers. </li></ul></ul>
  40. 40. <ul><li>The asynchronous pacemaker produces a pulse at a preset rate, for example 70 bpm, and delivers pulses to the heart regardless of the heart’s natural beating tendency and independent of the QRS complex. </li></ul><ul><ul><li>This pacemaker does not increase the heart rate in response to the body demand for more blood during exertion. </li></ul></ul>
  41. 41. <ul><li>However, a P-wave synchronous pacemaker does. </li></ul><ul><ul><li>The SA node depolarization responds to body demands through the vagus nerve and hormones transported in the blood. </li></ul></ul>
  42. 42. <ul><li>In a P-wave synchronous pacemaker, the SA node triggers the pacer, which in turn drives the ventricle. </li></ul><ul><ul><li>It is used when the AV node is blocked because of disease. </li></ul></ul>
  43. 43. <ul><li>As shown, this pacemaker requires two leads. </li></ul><ul><ul><li>The atrial lead feeds the atrial pulse back to a sensing amplifier. </li></ul></ul><ul><ul><li>The driver, connected to the ventricle, delivers the pacing pulse. </li></ul></ul>
  44. 44. <ul><li>The R-wave inhibited pacemaker also allows the heart to pace at its normal rhythm when it is able to. </li></ul><ul><ul><li>However, if the R-wave is missing for a preset period of time, the pacer will supply a stimulus. </li></ul></ul>
  45. 45. <ul><li>Therefore, if the pulse rate falls below a predetermined minimum, the pacemaker will turn on and provide the heart a stimulus. </li></ul><ul><ul><li>For this reason it is called a demand pacemaker. </li></ul></ul>
  46. 46. <ul><li>Another type of demand pacemaker uses a piezoelectric sensor shielded inside the pacemaker casing. </li></ul><ul><ul><li>When this sensor is slightly stressed or bent by the patient’s body activity, the pacemaker will automatically increase or decrease its rate. </li></ul></ul>
  47. 47. <ul><li>According to Medtronic, Inc., their model will react to a movement of one-millionth of an inch. </li></ul><ul><ul><li>It will change heart rates to as high as 150 bpm during vigorous activity or as low as 60 bpm during rest periods. </li></ul></ul>
  48. 48. <ul><li>A programmable pacemaker is one that can be altered both in its block diagram and in the size and rate of the pulse it delivers. </li></ul>
  49. 49. <ul><li>A pacemaker that can be reconfigured into four different block diagrams, after having been implanted. </li></ul>
  50. 50. <ul><li>A magnet may be placed over the pacemaker on the skin of the patient in order to activate a reed switch, which switches the pacemaker into one of the four configurations shown. </li></ul><ul><ul><li>Another kind of programming is done to alter the delivered stimulus and the pacemaker sensitivity to feedback signals. </li></ul></ul>
  51. 51. <ul><li>A programmable pacemaker is shown in the figure. </li></ul>
  52. 52. <ul><li>The telemetric programmer may be placed over the pacemaker to select pulse rates ranging from 30 to 155 bpm , feedback sensitivities from 0.7 to 4.5 V, pulse amplitudes from 2.5 to 10 V, and pulse widths from 0.25 to 1 ms, among other parameters. </li></ul>
  53. 53. <ul><li>A hard copy of the programming record is provided by the printer shown. </li></ul>
  54. 54. <ul><li>When temporary heart pacing is needed, an external pacemaker may be used. </li></ul><ul><ul><li>Since this device is not implanted, there is no need for extensive surgery. </li></ul></ul>
  55. 55. <ul><li>A temporary pacing lead uses a balloon tip, so that the flow of blood will carry the pacing electrode into the heart when the balloon is inflated. </li></ul>
  56. 56. The ECG Pattern and Cardiac Pacing <ul><li>The figure shows the appearance of the normal ECG signal as measured in the atrium. </li></ul>
  57. 57. <ul><li>Notice the large P wave, which is almost as high as the normal QRS complex. </li></ul>
  58. 58. <ul><li>In contrast, this figure shows the effect of adding a continuously operating pacemaker signal to the normal atrium. </li></ul>
  59. 59. <ul><li>Now the heart is responding only to the pacemaker, and the pacemaker is said to have “captured” the heart rate. </li></ul><ul><ul><li>Note that the QRS wave follows the pacemaker-generated P wave at a fixed interval, and that there are no beats generated sinoatrial (SA) node. </li></ul></ul>
  60. 60. <ul><li>The pacemaker signal is large enough and occurs at a high enough rate to keep the SA node in the depolarized condition so that it cannot fire. </li></ul><ul><ul><li>This is important, because an occasional, ectopic, SA-node beat would be entirely out of synchronization with the regularly occurring pacemaker beat. </li></ul></ul>
  61. 61. <ul><li>Eventually, a pacemaker-induced pulse would occur during the latter part of the QRS complex or during the T wave from the ectopic SA-node beat, and this would be trouble. </li></ul><ul><ul><li>It turns out that disease-weakened hearts are more sensitive than healthy hearts to any signal that arrive during the latter part of the QRS complex or the T wave, and such a weakened heart will go into fibrillation if a pacemaker beat and either of these signals happen to coincide. </li></ul></ul>
  62. 62. <ul><li>To avoid this hazard, the pacemaker signal is set large enough to preclude the occurrence of any inadvertent SA-node beats. </li></ul>
  63. 63. <ul><li>The above mode of pacemaker operation was always used when pacemakers were first invented, and it is still used if the P or QRS waves are weak, very irregular, or entirely absent. </li></ul><ul><ul><li>This mode has its problems in that no adjustment can be made for the normal change in heart rate from resting to exercise. </li></ul></ul>
  64. 64. <ul><li>Usually a rate of 70 heats per minute is used as a compromise. </li></ul><ul><ul><li>The requirement for continuous operation is reflected in reduced battery life, and the pacemaker has to be changed more often. </li></ul></ul>
  65. 65. <ul><li>If a patient has a more nearly normal heart, there may be no need for continuous pacing, and the unit is set in the “demand” mode. </li></ul><ul><ul><li>In this mode, the pacemaker detects the peak of the QRS wave and begins “counting.” </li></ul></ul>
  66. 66. <ul><li>If the next QRS wave occurs within what is called the “capture interval,” the pacemaker does nothing. </li></ul><ul><ul><li>If the QRS wave is late or absent, the pacemaker stimulates the heart. </li></ul></ul><ul><ul><ul><li>Here again, the locus of can be in the atrium or in the right ventricle, as necessary. </li></ul></ul></ul>
  67. 67. <ul><li>If the QRS wave stops entirely, the pacemaker will stimulate the heart at about 70 beats per minute; </li></ul><ul><ul><li>One might say that it switches from the “demand” mode to the “continuous” mode. </li></ul></ul>
  68. 68. <ul><li>Demand operation results in longer battery life and allows the patient to benefit from the normal heart-rate control system that adjusts the beat to the demands of the body. </li></ul><ul><ul><li>The pacemaker is available for action if and when the patient’s own heart-rate control system should fail. </li></ul></ul>